Conventionally, an information reading apparatus for reading information recorded on the above-described type of multilayer holographic recording medium is known such as described in Japanese Patent Laid-Open Publication No. 2000-149318, for example. In this apparatus, a light source and an objective lens are moved so as to make light incident on a desired recording layer and a focusing optical system, an expanding optical system, and a two-dimensional photodetector are integrally moved up and down, thereby selectively performing reproduction for each recording layer of multilayer type hologram.
Japanese Patent Laid-Open Publication No. 2000-149318 also discloses an apparatus that can selectively perform reproduction for each recording layer only by moving a correction optical system up and down without moving the focusing optical system, the expanding optical system, and the two-dimensional photodetector up and down.
That is, the information reading apparatus described in Japanese Patent Laid-Open Publication No. 2000-149318 requires either integral upward and downward movement of the focusing optical system, the expanding optical system, and the two-dimensional photodetector or upward and downward movement of the correction optical system only without moving the focusing optical system, the expanding optical system, and the two-dimensional photodetector up and down. Therefore, that information reading apparatus has limits on improvement of recording density and a data transfer rate because of precision and a rate of the upward and downward movement of these elements.